In recent years, as represented by mobile telephones or digital broadcasting, a digital modulation method by modulating digital signals for transmission and reception is widely used as a radio communication method.
The digital modulation mainly includes Phase Shift Keying (PSK) modulation, Quadrature PSK (QPSK) modulation, Quadrature Amplitude Modulation (QAM), and Orthogonal Frequency Division Multiplexing (OFDM).
Each of the above modulation methods is a communication method by use of a signal, in which an I signal (In-phase signal, In-phase component) and a Q signal (Quadrature signal, Quadrature component) are quadrature multiplexed.
In IQ modulation or IQ demodulation by use of these I and Q signals, the following method is simple and effective as one of the methods. That is, the simple and effective method is to directly modulate a baseband signal represented as IQ with a mixer circuit or directly demodulating a high-frequency signal that is IQ quadrature modulated, by use of an IQ quadrature local signal having two signals of carrier frequencies different from each other by 90 degrees. However, in this direct modulation or direct demodulation, if the phase difference between the I local signal and Q local signal deviates from 90 degrees, which is an ideal phase difference, the IQ quadrature property is damaged and the communication quality is degraded.
For example, if the phase difference between the IQ quadrature local signals deviates from exact 90 degrees in a transmitter or receiver for IQ quadrature modulation, IQ constellation of the modulation signal and demodulation signal would distort and lead to the deterioration of Error Vector Magnitude (EVM). Thus, it is important to suppress the phase error of IQ quadrature local signals to be small for achieving a superior communication quality. However, the phase error of the IQ quadrature local signals is caused by a variety of factors, such as incompleteness of a 90-degree phase shift circuit for generating the I and Q local signals, unbalance on the I-side or Q-side transmission paths of the I and Q local signals, or the like. For this reason, in order to achieve an extremely small phase error, the phase correction of the IQ local signals is needed.
The measure for adjusting the phase error of the IQ local signals is described in Patent Document 1, for example. In Patent Document 1, the phase difference of the IQ local signals is adjusted to suppress the phase error by use of a circuit as shown in FIG. 18. In this circuit, a local signal is input into a single input terminal, and the local signal in which its phase has been adjusted is obtained from a differential output port. Then, the capacitance value and the resistance value are respectively adjusted with capacitances C1 and C2 or resistances R1 and R2 being as variable elements, so that the phase of the signal appearing at the differential output port is changed according the value.